1. Field of the Invention
The present invention relates to a semiconductor device and, more specifically, to a structure of a wiring substrate on which a plurality of semiconductor elements having connecting electrodes such as projecting electrodes are mounted and which is capable of moderating an unpredictable deformation due to external force or impact.
2. Description of the Related Art
A semiconductor device is generally constituted by a plurality of semiconductor elements each including an IC, an LSI, or like formed on a semiconductor substrate, and a wiring substrate on which the semiconductor elements are mounted. The semiconductor elements are packaged by using a package of synthetic resin, ceramic, etc. in order to protect them from mechanical destruction and contaminations such as dust, chemical, gas, and moisture that adversely affect the characteristics of the semiconductor substrate. To mount a semiconductor element on the wiring substrate, conventionally, a lead attached to the semiconductor element by means of a lead frame, is connected to a circuit pattern of a wiring layer of the wiring substrate. Recently, the semiconductor devices have been used versatilely, and the density of semiconductor elements mounted on a wiring substrate has been increased. Furthermore, a thin wiring substrate of 1 mm or less has recently been used for a memory card, and the number of memory elements mounted on the memory card tends to increase.
FIGS. 1, 2A and 2B show a conventional card type semiconductor device such as a memory card. The semiconductor device includes a plurality of semiconductor elements 2 each constituted by a resin package having a semiconductor substrate on which an integrated circuit is formed, and the semiconductor elements 2 each have leads 21 electrically connected to the integrated circuit and extending outside the resin package. The semiconductor elements 2 are mounted on a wiring substrate 10. The leads 21 of the semiconductor elements 2 are laid on a circuit pattern of a wiring layer 11 formed on the surface of the wiring substrate 10 to be connected with the wiring layer by solder or the like. If external force or impact is exerted on the wiring substrate 10 mounted with the semiconductor elements 2, in the direction of an arrow shown in FIG. 2B, the substrate 10 is deformed, and stress is generated at connecting portions between the semiconductor elements 2 and wiring substrate 10. The thinner the wiring substrate, the greater the deformation and stress. Since, however, the relatively long leads 21 are interposed between the semiconductor elements 2 and wiring substrate 10, the stress is absorbed by deformation of the leads 21, and the reliability of the connecting portions is maintained. In the card type semiconductor device such as a memory card, the thickness of the wiring substrate is decreased more and more, and the mounting density is improved.
There is a limit to the conventional semiconductor elements in improving the mounting density, since the leads greatly project from the semiconductor elements. For this reason, as shown in FIGS. 3, 4A and 4B, the semiconductor elements 2 have projecting electrodes 22 and are mounted on the wiring substrate 10. More specifically, a plurality of projecting electrodes 22 constituted by, e.g., solder are formed on the surface of the wiring substrate 10, and each electrically connected to the integrated circuit (not shown) therein. The projecting electrodes 22 are placed on the circuit pattern of the wiring layer 11 formed on the surface of the wiring substrate 10, and connected to the wiring layer by pressure and heating. The height of each projecting electrode 22 is 100 .mu.m or less, and connecting electrodes such as bump electrodes, each of which has a greater width relative to the height thereof, are used as the projecting electrodes. If external force or impact is exerted on the wiring substrate 10 mounted with the semiconductor elements 2, in the direction of an arrow shown in FIG. 4B, the substrate 10 is deformed, and stress is generated at connecting portions between the semiconductor elements 2 and wiring substrate 10. The thinner the wiring substrate, the greater the deformation. The stress causes the semiconductor element 2 to be broken as indicated by numeral 30 of FIG. 4B, and causes the projecting electrode 22 to be separated from the wiring substrate 10 as indicated by numeral 40 of FIG. 4B.
The structure of the semiconductor device shown in FIGS. 3, 4A and 4B is effective in increasing the semiconductor elements mounted on the wiring substrate and making the wiring substrate thinner. If, however, the external force is applied to the substrate in the direction indicated by the arrow of FIG. 4B, the semiconductor substrate is broken (30) or separated (40) from the wiring substrate, since it does not have any equivalents for those package leads of the semiconductor device shown in FIGS. 1, 2A and 2B which absorb the deformation of the wiring substrate. In particular, the card type semiconductor device is thinned more and more. The thinner the device is, the more easily it is deformed. In a portable device such as a memory card, its reliability is inevitably degraded because of unpredictable external force or impact exerted on the device.